The economic reality forces a rapid evolution of the wind energy business: upscaling of wind turbine designs, wind farms implemented in complex terrain, in limited concessions with higher power densities, off-shore installations . . . . A future challenge will also exist in the flexibility enhancement of wind farms through an increased ability to offer ancillary grid services. These evolutions imply continuous challenges for the design, operation and maintenance of wind turbines. The high and rapidly changing loads cause important stresses on the wind turbine components, that need to be accounted for in the design phase. A reliable operation and maximum availability is necessary to comply with the small economical margins related to the investments. The maintenance of the wind turbines is a particular challenge, due to the increasing costs of the hardware and the complex logistic issues linked with the interventions, especially on offshore installations.
All these challenges require powerful support tools, amongst which condition monitoring plays a prominent role, as an aid in improving the ratio between predictive and corrective maintenance. In a wind turbine, a complex interaction exists in different energy conversions: kinetic wind energy is transferred through the blades into a shaft torque on the drivetrain. The generator converts this mechanical energy into electrical energy. Actual condition monitoring approaches typically try to identify important parameters for these individual technological domains. A multidisciplinary approach is however needed in order to increase the capabilities.
An improved monitoring of the impact of load variations on the drivetrain of a wind turbine is needed. The resulting shaft torque variations have an important impact on the gearbox cogwheels and the bearings of the shaft-line components, while the electrical power variations induce thermal stresses for the electrical components. The electrical components are increasingly important in new design concepts. Due to the tendency towards a full converter technology in the shaft train design, i.e. eliminating or simplifying the gearbox section significantly, thereby enabling lighter and more robust wind turbines, more direct drive technology will occur, wherein the electrical components require heavier load. Moreover, the impact of failures of electrical components becomes even more important, considering offshore or highly remote systems.
Mechanical shaft torque variations can be measured using the input of strain gauges on the rotor or by measuring angular shaft positions as a function of time at 2 or more locations on the drivetrain of a wind turbine, as e.g. referred to in EP 2498076 A1 and EP 2684018 A1. Other techniques also exist (e.g. magnetostrictive sensors), but are still considered as less robust or less applicable in a generic way.
The use of strain gauges is the most complete measurement because it gives an indication of both the cause (shaft torque variations) and the effect (strain) of load variations. They are however expensive to install and maintain, and not robust for a permanent use, unless they would be embedded in the design of the shaft train. A calibration of the measurements with actual load (e.g. electrical measurements) is needed if one wants to use them as an absolute indicator of shaft torque.
An accurate measurement of angular shaft positions as a function of time on 2 or more locations on the shaft train enables a more robust and cost effective measurement of shaft torque variations. The input is determined by pulses generated by shaft encoders or toothed wheels. Instantaneous variations of the angular shaft position at the measurement locations cause variations of the pulse spacing in each pulse train signal. A detailed data processing enables a qualitative assessment of both the dynamic and static components of shaft torque variations. Here also, a calibration with another measurement technique (e.g. electrical measurements) is required for a more quantitative assessment. The main disadvantage of angular shaft position variation measurements is the high speed data sampling, requiring specific measurement technology, and the consecutive need for high data storage volumes if one wants to calculate the angular shaft position variations through post-analysis.
Electrical measurements (currents and voltages measured at the generator) are another way of evaluating load variations on the shaft train. These measurements have the advantage of being available in most standard measurement setups and also facilitate an easy scaling of dynamic load variations with respect to static loads. The resulting shaft torque variations on the generator shaft can be deduced by using a theoretical model of the electrical energy conversion chain, i.e. the combination of the generator and the electrical power converter. The variation of the generator shaft torque is representative for the variation on the complete shaft train for a gearless wind turbine configuration or if one disregards the torque losses in the gearbox (which is an acceptable assumption if one is interested in the relative magnitude of the peak shaft torque with respect to nominal torque). Electrical measurements have however the important disadvantage to be unavailable when the coupling with the grid is opened, as is the case during many important transients (e.g. an emergency stop). Unfortunately, there is a particular interest of capturing load variations in such situations, in order to assess their impact on the lifetime of the drivetrain components of a wind turbine.